The present invention relates to a fluid friction clutch and more especially to a fluid friction clutch of the type having two working surfaces separated by a gap which is adapted to receive a viscous fluid in a controlled manner, such as by means of a valve which is controllable as a function of temperature.
Such a fluid friction clutch is known, for example, from German Patent No. 12 84 186. In clutches of this type the torque is transmitted between the drive disk on the primary side and the housing on the secondary side, carrying for example a blower, by the shear forces of a viscous fluid present in the gap between the drive disk and the clutch housing. The magnitude of the torque which can be transmitted depends on the amount of working medium present in the two gaps between the drive disk and the clutch housing, i.e., the degree of filling. In order to prevent metal-to-metal contact, i.e., scoring between the drive disk and the clutch housing, such known drive disks are frequently equipped with non-metallic linings. Between these segment-like linings, channels are provided for the radially outwardly directed flow of the working medium, whereby the uniform distribution of the working medium within the gap is obtained even with partial degrees of filling. Furthermore, cork linings in particular have the capability of becoming saturated with the working medium and thus contribute to the assurance of a uniform distribution of the working medium. Such linings, however, are not advantageous from an economical standpoint, since they generate additional costs.
For this reason, as is known from German Auslegeschrift No. 26 02 608, friction linings of this type have already been eliminated. This, however, has resulted in disadvantages affecting the rotational speed behavior of the clutch during the engagement phase of the clutch at a certain temperature. Attempts have been made to eliminate these undesirable rpm fluctuations which are accompanied by the generation of disagreeable noise, by providing a temperature sensitive magnetic switch, which leads to the sudden opening of the valve for the working medium. But this type of black/white switching is not always desirable, as it leads to the abrupt engagement of the clutch and thus to torque impacts.
Known fluid friction clutches having smooth drive disks without linings, in addition to the abovementioned disadvantage of rotational speed fluctuation in the engagement phase, i.e., when partially filled, have the disadvantage that their engagement and disengagement are delayed. The delayed engaging of the clutch, when used for fans in combustion engines, can lead to overheating of the engine. The delayed disengagement of the fan increases the fuel consumption of the vehicle and extends the period of objectionable noise generation.